The present invention relates to ferromagnet-insulator-ferromagnet junction devices using a ramp-edge geometry. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Doped manganites (RxM1xe2x88x92x)MnO3 where R is a rare earth element such as La, Pr, and Nd and M is a divalent element such as Ba, Sr, Ca and Pb, have been the topic of intense scrutiny in recent years because of the large magnetoresistance (MR) that occurs near their metal-insulator transition temperature. Even though MR, [(R(O)xe2x88x92R(H)/R(O)], values of nearly 100% are obtained in the manganite thin films, a magnetic field of several tesla is required and it is only seen at low temperatures (T less than 100 K). Hence, it is questionable whether the doped manganites would ever be useful for low field sensors in a single layer form. Recently, Sun et al., Appl. Phys. Lett., v. 69(#22), pp. 3266-3268, (1996) and Appl. Phys. Lett., v. 70(#13), pp. 1769-1771 (1997) have demonstrated the existence of large magnetoresistance at low fields in the doped manganites based on trilayer sandwich junctions (For magnetoresistance with conventional electrodes, see also, U.S. Pat. Nos. 5,801,984; 5,835,314; and, 5,841,692). Their junctions are based on the spin-dependent tunneling device utilizes the uneven spin distribution of conduction electrons at the Fermi level in ferromagnetic metals. The junction resistance depends on the relative orientation of the magnetization directions in electrodes. The change of the tunnel resistance (xcex94Rj=Rj (H)xe2x88x92Rj) in a field is given by xcex94Rj/Rj=2P1P2/(1+P1P2) where Rj is a junction resistance when the direction of magnetization is parallel, and P1 and P2 are the spin polarizations of the two ferromagnetic electrodes.
Since the spin polarization in the doped manganites is believed to be larger than that in conventional ferromagnetic metals due to the half-metallic nature, these materials have potential as a ferromagnetic metal electrode in devices using spin-dependent transport effects. Hence, compared to the tunneling junctions based on the conventional ferromagnet metal electrodes, MR in the tunneling junctions using the manganites are expected to be larger.
As learned from the study of thin-film high temperature superconductor applications that the control of interfaces in metal-oxide heterostructures is quite difficult. In particular, the fabrication of vertical sandwich structures has been complicated by the large pinhole density, the existence of particulates during the growth process, and the need for a large junction area due to limitations of the lithography process. As was used in a Josephson junction design, a ramp-edge structure can have technical advantages in metal-oxide based junction devices due to a small junction area by nature of the design, see Jia et al., Appl. Phys. Lett., v. 71(#12), pp. 1721-1723, (1997).
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides an edge-geometry magnetic tunnel junction device including a substrate, a first ferromagnetic electrode upon said substrate, a spacing layer of an insulating or highly resistive material upon a portion of said first ferromagnetic electrode, said first ferromagnetic electrode and said insulating or highly resistive material further characterized as having one side of this combination configured in a ramp edge shape, a thin layer of an insulating material upon the ramp edge of said combination of said first ferromagnetic electrode and said insulating or highly resistive material, and, a second ferromagnetic electrode upon said thin layer of insulating material.
The present invention further provides a method for preparing a magnetic tunnel junction device including forming a first ferromagnetic electrode upon a substrate, depositing a spacing layer of a material characterized as insulating or highly resistive at operating temperatures of said device, said material having chemical and structural compatibility with the first ferromagnetic electrode to form an intermediate composite structure, depositing a photoresist material upon a portion of said intermediate composite structure, etching off selected areas of said intermediate composite structure, removing said photoresist material to yield an etched intermediate composite structure, depositing a thin layer of a material characterized as insulating at operating temperatures of said device, upon selected areas of said etched intermediate composite structure, and, depositing a second ferromagnetic upon the thin layer of insulating material.